第三篇  The Exploding Lakes of Cameroon 
     What comes to mind when you think of a lake? You probably imagine a pretty scene with blue water，birds，and fish．For the people in the northwestern Cameroon，however, the image is very different.  For them，lakes may mean terrible disasters.  In 1984，poisonous gases exploded out of  Lake Monoun and came down into the nearby villages，killing thirty—seven people．Two years later,Lake Nyos erupted A cloud of gases rolled down the hills and into the valleys and
killed 1,700 people． 
     Lake Nyos and Lake Monoun are crater(火山口)lakes They were formed when water collected in the craters of old volcanoes The volcanoes under Lake Nyos and Lake Monoun are not active anymore.  However, poisonous gases from the center of the earth continue to flow up through cracks in the bottom of the lake. This is normal in a crater lake. In most crater lakes，these gases are released often because the water‘turns over’regularly．That is，the water from the bottom of the lake rises and mixes with the water at the top，allowing the gases to escape slowly． 
     However，in Lakes Nyos and Monoun，there is no regular turning over．No one knows the reason for this fact，but as a result，these lakes have more gases tapped at the bottom than other crater lakes.  In fact，scientists who have studied Lakes Nyos and Monoun have found 16,000 times more gases．When a strong wind，cool weather a storm，or a landslide(滑坡)causes the water to turn over suddenly，the gases escape in a violent explosion． 
     In the past，no one knew when the gases might explode，so there was no way for the villagers to escape disaster.  Now scientists from the United States，France，and Cameroon have found a way to reduce the gas pressure at the bottom of Lake Nyos．They stood a 672-foot plastic pipe in the middle of the lake，with one end of the pipe near the bottom and the other end in the air．Near the top of the pipe，the team put several holes that could be opened or closed by a computer．Now，when the gas pressure gets too high，the holes are opened and some of the gas-filled water shoots up through the pipe into the air like a fountain．With less pressure，a disastrous explosion is much less likely．However, the scientists are not sure that one pipe will be enough to prevent explosions．They hope to put in others soon and they plan to install a similar pipe and a computer system at Lake Monoun as well．
     To protect people nearby until all of the pipes are in place．the scientists have installed early warning systems at both lakes.  If the gas pressure rises to a dangerous level，computers will set off loud sirens(警报)and bright lights to warn the people in the villages．That way, they will have time to escape from the dangerous gases．

41 What will happen when Lake Nyos and Lake Monoun explode?
A  Water will flow down the hills
B  Poisonous gases will be released suddenly．
C  A strong wind will rise from the lakes
D  The volcanoes will come to life．

42 Which of the following statements about Lake Nyos and Lake Monoun is true?
A  They were formed in 1984．
B  They are at the top of two active volcanoes．
C  They are not like most other crater lakes．
D  Water in them turns over regularly．

43 Lake Nyos and Lake Monoun explode because
A  the gases rise to the top and mix with air．
B  people from the villages turn over the water．
C  scientists have put in a computer system．
D  they have more gases trapped at the bottom than other crater lakes

44 A team of scientists has
A  erected a pressure-releasing pipe in the lake
B  identified the gases at the bottom of the lake
C  built a beautiful fountain near the lakes
D  removed all dangerous gases from the lakes

45 What do we learn from the last paragraph?
A  Scientists are planning to install pipes in all crater lakes．
B  Scientists still do not know how to prevent gas explosions
C  Explosion disasters could be avoided in the future
D  Warning systems have been set up in the villages nearby．